1. Field of the Invention
The present invention relates to nickel-coated substrates. The invention has particular applicability to the manufacture of magnetic recording media.
2. Description of the Background Art
Nickel (Ni) plating, particularly electroless Ni platings or deposits, enjoys technological applicability in various industries, such as the electronic, oil and gas, aerospace, machinery, automobile and magnetic recording media industries. For example, in magnetic recording media applications, a magnetic disc comprising a non-magnetic substrate such as aluminum (Al) or an aluminum alloy may be coated with an amorphous nickel deposit.
Magnetic disk drives are normally operated using a contact start-stop (CSS) method. In the CSS method, a head begins to slide against the surface of the magnetic disk as the disk begins to rotate and, upon reaching a predetermined rotational speed, the head floats in air a fixed distance above the surface of the disk. The distance that the head floats above the surface of the disk is called the flying height. The head floats above the surface of the disk due to dynamic pressure effects caused by the air flow generated between the recording operations of a disk drive, the head is maintained at a controlled distance from the surface of the magnetic disk, supported on a bearing of air as the disk rotates. Upon terminating operation of the disk drive, the rotational speed of the magnetic disk is decreased such that the head begins to slide against the surface of the disk, until it eventually stops, in contact with and pressing against the disk. Thus, each time the head and disk assembly is driven, the sliding surface of the head repeats this cyclic operation consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk and stopping.
In order to achieve high areal density for magnetic disk drives, it is considered necessary to minimize the flying height of the head above the surface of the magnetic disk. One technique for minimizing the flying height of the head above the surface of the magnetic disk uses substrates having an extremely smooth, defect-free surface for fabricating such magnetic disks. The absence of defects such as, for example, pits is especially important, since pits may adversely affect the writing of information to the magnetic disks.
For manufacturing magnetic recording media, amorphous nickel plating is conventionally applied to a non-magnetic substrate, such as, for example, aluminum (Al) or an Al-alloy substrate. Electroless plated NiP is typically chosen because it exhibits desirable physical and chemical properties, such as hardness, lubricity, appearance, and corrosion resistance.
It is recognized, however, that electroless metal plating, such as electroless NiP plating known to the art, does not achieve coatings exhibiting a desired degree of surface smoothness, particularly the degree of smoothness necessary to satisfy the high areal recording density objectives of current magnetic recording media (e. g., an average surface roughness (Ra) less than about 30 Å). Market competitiveness further requires achievement of ultra-smooth electroless nickel coatings on non-magnetic substrates with increased manufacturing throughput and higher yield.
Conventional techniques for improving the surface smoothness of plated nickel coatings include the incorporation of a polishing step subsequent to the electroless metal plating. Polishing, however, requires a considerable capital investment and results in reduced process throughput.
Accordingly, there exists a need for as-deposited ultra-smooth nickel coatings having reduced defects that do not require subsequent polishing. There exists a particular need for methodology enabling the deposition of amorphous nickel coatings on non-magnetic substrates, which have an as-deposited surface roughness (Ra) less than about 30 Å.